The invention relates to a detector of sudden changes or discontinuities in a medium moving relative to the detector and, more particularly, to such a detector for flowing fluids in medical diagnostic and therapeutic instruments.
It is desirable in many situations to detect a sudden change or discontinuity moving relative to a detector. Such discontinuities include a liquid plug forced along a gas conduit, a gas bubble forced along or rising up a liquid conduit, or a change from one flowing liquid or gas to another. These discontinuities can damage apparatus at the end of the conduit or interfere with the intended use of the fluid in exemplary ways described below for medical diagnostic and therapeutic instruments. A host of other discontinuities can also be desirably detected, however, in other arts such as aerial reconnaisance or character recognition, for example.
Medical diagnostic and therapeutic instruments offer good examples of the problems presented by fluid discontinuities. Mass spectrometers or discrete gas analyzers, for example, are desirable diagnostic tools for patients in intensive care or operating theatres. Samples of the patient's breath are drawn into the instrument through a small, capillary-like tube inserted into one of the patient's breath passages. The samples of the patient's breath are then analyzed in the instrument for carbon dioxide or anesthesia content to help diagnose the patient's condition. The patient's breath passages are moist, however. The capillary tube thus frequently fills with a short plug of liquid drawn into the tube with the gas or condensed in the tube as the gas flows to the instrument. If such liquid reaches the instrument it can damage it or interfere with its operation. This problem is so severe that it has restricted the desired use of such gas diagnostic instruments.
Intra-veinous fluid supplies illustrate the problem of fluid discontinuities in medical therapeutic instruments. They are frequently used to supply liquids to a patient's blood stream. It is well known, however, that a gas bubble entering the blood stream with the fluid can be dangerous and even fatal to the patient. The instruments are therefore carefully designed to prevent bubbles from forming in the fluid supply and to trap any bubbles which do form, but generally are not equipped to stop the flow of fluid to the patient if a bubble should escape into the fluid.
There are many other examples in medical and chemical process instrumentation in which it is desirable to detect a discontinuity in a flowing fluid. The diverse desire for such a detector has prompted prior efforts at making one.
One prior effort for a medical respiratory-gas analyzer is disclosed in U.S. Pat. No. 4,197,858 issued Apr. 15, 1980 to John J. Osborn. In the patent, gas to be analyzed is drawn into a chamber through a tube from a patient's breath passage. The gas is then withdrawn form the top of the chamber for analysis while liquid drawn in with the gas settles to the bottom of the chamber. When the liquid reaches a level where it contacts a pair of probes, it changes the electrical conductivity between the probes to trigger a sensing circuit. The sensing circuit then closes a valve in the gas line to the analyzer to prevent the fluid from entering the analyzer and opens a valve to a gas pressure supply to push the liquid from the chamber back along the tube into the patient.
The chamber poses three problems. First, even though it is small, it necessarily permits some mixing of the gas drawn into the chamber at one instant with gas previously drawn in. The analysis of the gas drawn out of the chamber thus represents a mixture of the patient's present condition with his prior condition. Second, the chamber requires sterilization or replacement before the apparatus can be used again because liquid contaminated by a prior patient would otherwise be pushed back into the next patient. Either sterilization or disposal and replacement of the chamber add to the cost and difficulty of using the device. Third, the chamber is position sensitive. If it is improperly set up on its side, or falls over, the liquid may contact the probes supposed to be at the top of the chamber to trigger the purging-pressure response, but not cover the return path to the patient to be purged by the pressure.
Another prior effort is disclosed in U.S. Pat. No. 3,257,782 issued June 28, 1966 to E. L. Weiss. It avoids the mixing problem of the chamber by using discrete electrically-conducting sections as part of a gas supply line. If liquid bridges insulation between a first pair of the conducting sections, an alarm is triggered by conductivity of the liquid. If the liquid progresses further along the line to bridge a second pair of the conducting sections, the conduction of the liquid closes an outlet value for the gas. This arrangement, however, does not avoid the problem of sterilizing or replacing the conducting sections which would arise if the device were used in many medical instruments such as the described respiratory gas analyzer in which the liquid is forced back into the patient.